The purpose of this study is to evaluate in Chronic Kidney Disease (CKD) patients not on dialysis and who have an Fibroblast growth factor 23 (FGF23) serum levels elevated, the effect of non calcic phosphate binder: sevelamer carbonate. This treatment could lead to a diminution of FGF23 serum levels due to the diminution of intestinal absorption of dietary phosphate. In addition, the investigators will describe the impact of the FGF23 level monitoring on the main phosphocalcium metabolism markers as phosphatemia, intact parathyroid hormone (iPTH), serum calcitriol and phosphaturia.